Currently, in the field of non-aqueous electrolyte secondary batteries, studies on the lithium ion secondary batteries having a high voltage and a high energy density are being actively developed. In typical lithium ion secondary batteries, lithium-containing transition metal oxides such as LiCoO2 are used as the positive electrode active materials, carbon materials are used as the negative electrode active materials, and porous films made of polyethylene or polypropylene are used as the separators. A non-aqueous electrolyte generally includes a non-aqueous solvent and a solute dissolved therein. As the non-aqueous solvent, for example, a cyclic carbonic acid ester, a chain carbonic acid ester, a cyclic carboxylic acid ester and the like are used; as the solute, for example, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4) and the like are used.
For the purpose of improving the battery performances, the improvement of the positive electrode active material, negative electrode active material, separator and non-aqueous electrolyte has hitherto been attempted. As for the separator, for example, the following improvements have been developed.
Japanese Patent No. 3048083 (Document 1) has proposed that there is used as a separator a film laminate formed of a porous fluorocarbon resin film including polytetrafluoroethylene (PTFE) and the like and a polyethylene film or a polypropylene film; by laminating a fluorocarbon resin film having a high melting point on a polyethylene film or a polypropylene film, the melting of the separator at the time of abnormal heat generation can be prevented. Consequently, the safety of the battery in short-circuiting or abnormal use can be improved.
Japanese Laid-Open Patent Publication No. 5-258741 (Document 2) has proposed that a separator composed of two layers different in pore size from each other is used for the purpose of improving the safety of the batteries using lithium metal as the negative electrode active material. The layer smaller in pore size suppresses the dendritic growth of lithium metal, and consequently the internal short circuit at the time of charge/discharge and the burning concomitant therewith can be suppressed. It is to be noted that Document 2 discloses a separator formed by laminating a polytetrafluoroethylene film and a film smaller in pore size made of polypropylene.
As for the non-aqueous electrolyte, the following improvements, for example, have been developed.
Japanese Patent No. 3396990 (Document 3) has proposed the use of a non-aqueous solvent including a mixture composed of thiophene dioxide and an acyclic sulfone. Thiophene dioxide forms a protective coating on the negative electrode carbon, and the damage of the negative electrode carbon concomitant with the charge/discharge cycle is thereby suppressed. The addition of the acyclic sulfone decreases the viscosity of the non-aqueous electrolyte, and the ionic conductivity of the non-aqueous electrolyte is thereby improved. Thus, the battery cycle properties can be improved.
Japanese Patent No. 3239267 (Document 4) has proposed that a mixture composed of a cyclic carbonic acid ester and acetonitrile is used as a non-aqueous solvent. Acetonitrile decreases the viscosity of the non-aqueous electrolyte, and the ion conductivity of the non-aqueous electrolyte is thereby improved. Further, acetonitrile is highly resistant to oxidation, so that the deterioration of the non-aqueous electrolyte can be suppressed even when a positive electrode active material having a high electric potential is used. Thus, the high-rate discharge characteristics and the cycle characteristics of the battery can be improved.
Japanese Laid-Open Patent Publication No. 2005-340223 (Document 5) has proposed that a nonflammable fluorine-containing ether is used as a non-aqueous solvent for the purpose of improving the battery safety.
It has been known that the lithium-containing transition metal oxide as the positive electrode active material undergoes an intensive elution of the metals constituting the metal oxide when the battery is stored at high voltages and at high temperatures. In this connection, even when there is used a separator as proposed in above Documents 1 and 2 formed by laminating a polyethylene film or a polypropylene film and a polytetrafluoroethylene film, the elution of the metal atoms from the lithium-containing transition metal oxide cannot be suppressed. Consequently, the metal atoms eluted from the lithium-containing transition metal oxide are deposited on the negative electrode to cause the impedance increase of the negative electrode, the clogging of the separator and others. Thus, a battery containing a lithium-containing transition metal oxide as the positive electrode active material undergoes a degradation of the rate characteristics after storage.
Even when such non-aqueous solvents as proposed in Documents 3 to 5 are used, such above-described elution of the metal atoms from the lithium-containing transition metal oxide cannot be suppressed. Consequently, as described above, the rate characteristics after storage is degraded.
Accordingly, an object of the present invention is to provide a non-aqueous electrolyte secondary battery capable of alleviating the degradation of the rate characteristics when the battery is stored, in particular, at high voltages and at high temperatures.